diff --git a/GNUmakefile b/GNUmakefile
index 88f4ff64d..22b1a65b7 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -1,4 +1,5 @@
 
+export SOURCE_PORT = $(shell which chocolate-doom)
 export SDL_VIDEODRIVER = dummy
 export DOOMOPTS = -mb 16 -nodraw -noblit -nosound \
                   -noautoload -nogui -nograbmouse
diff --git a/testrunner b/testrunner
index 777d97cdd..e838ea68c 100755
--- a/testrunner
+++ b/testrunner
@@ -20,11 +20,13 @@ def read_config(path):
 
     return result
 
+source_port = os.getenv("SOURCE_PORT")
+assert source_port != "", "SOURCE_PORT environment variable not set."
 lmp_file, out_file = sys.argv[1:]
 cfg = read_config(lmp_file)
 
 args = [
-    "chocolate-doom", "-iwad", cfg["iwad"],
+    source_port, "-iwad", cfg["iwad"],
     "-statdump", out_file,
     "-timedemo", lmp_file,
 ] + shlex.split(os.getenv("DOOMOPTS", ""))